The present invention relates to carbon foam and composites, and more particularly to a process for producing them.
The extraordinary mechanical properties of commercial carbon fibers are due to the unique graphitic morphology of the extruded filaments. See Edie, D. D., “Pitch and Mesophase Fibers,” in Carbon Fibers, Filaments and Composites, Figueiredo (editor), Kluwer Academic Publishers, Boston, pp. 43–72 (1990). Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a pitch precursor can be considered to be an interconnected network of graphitic ligaments or struts, as shown in FIG. 1. As such interconnected networks, they represent a potential alternative as a reinforcement in structural composite materials.
Typical processes for producing carbon foams utilize a blowing technique to produce a foam of the pitch precursor in which the pitch is melted and passed from a high pressure region to a low pressure region. Thermodynamically, this produces a “Flash,” thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, “Novel Hybrid Composites Based on Carbon Foams,” Mat. Res. Soc. Symp., Materials Research Society, 270:29–34 (1992), Hagar, Joseph W. and Max L. Lake, “Formulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor,” Mat. Res. Soc. Symp., Materials Research Society, 270:35–40 (1992), Gibson, L. J. and M. F. Ashby, Cellular Solids: Structures & Properties, Pergamon Press, New York (1988), Gibson, L. J., Mat. Sci. and Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36(4), (1976), and Bonzom, A., P. Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Then, the pitch foam must be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and “setting” the pitch so it does not melt during carbonization. See Hagar, Joseph W. and Max L. Lake, “Formulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor”, Mat. Res. Soc. Symp., Materials Research Society, 270:35–40 (1992) and White, J. L., and P. M. Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required. The “set” or oxidized pitch is then carbonized in an inert atmosphere to temperatures as high as 1100° C., followed by subjection to temperatures as high as 3000° C. to produce a graphitic carbon foam.
Other techniques utilize a polymeric precursor, such as phenolic, urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries for Open-Celled Foams,” Mat. Res. Soc. Symp., Materials Research Society, 270:41–46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117–127 (1990), Cowlard, F. C. and J. C. Lewis, J. of Mat. Sci., 2:507–512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285–302, (1969). High pressure is applied and the sample is heated. At a specified temperature, the pressure is released, thus causing the liquid to foam as volatile compounds are released. The polymeric precursors are cured and then carbonized without a stabilization step. However, these precursors produce a “glassy” or vitreous carbon which does not exhibit graphitic structure and, thus, has low thermal conductivity and low stiffness. See Hagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries for Open-Celled Foams,” Mat. Res. Soc. Symp., Materials Research Society, 270:41–46 (1992).
In either case, once the foam is formed, it is then bonded in a separate step to the facesheet used in the composite. This can be an expensive step in the utilization of the foam.
The process of this invention overcomes these limitations, by not requiring a “blowing” or “pressure release” technique to produce the foam. Furthermore, an oxidation stabilization step is not required, as in other methods used to produce pitch-based carbon foams. This process is less time consuming, and therefore, will be lower in cost and easier to fabricate. Moreover, the foam can be produced with an integrated sheet of high thermal conductivity carbon on the surface of the foam, thereby producing a carbon foam with a smooth sheet on the surface to improve heat transfer.
The present invention further relates to a thermally-conductive foam material derived from carbonaceous precursor, and more particularly to a thermally conductive, pitch-derived carbon foam having high thermal conductivity and heat exchanging properties.
The removal of unwanted heat is a frequently encountered problem. Conventional solutions include cooling fans, ice packs and refrigeration systems. In the latter, a working fluid is compressed (condensed) and pumped into an expansive chamber or pipe system where it evaporates, pulling heat from the atmosphere to satisfy its needed latent heat of vaporization, and thus cooling the surrounding environment. Air blown through the heat exchanger may be cooled and circulated to cool larger volumes such as in domestic and automotive air conditioning systems.
Active cooling (refrigeration) typically requires complex equipment including pumps, valves, compressors, etc. Many refrigeration systems require the use of CFCs (Freon), which is considered hazardous or environmentally unfriendly. An evaporative cooling system with a high thermal conductivity medium would offer a simpler, lower cost alternative. There is a need for portable coolers which are lightweight and inexpensive so as to be deployed in the field or in third world countries.
The thermally conductive carbon foam of this invention overcomes the limitations of the prior art.
In addition, the present invention relates to porous carbon foam filled with phase change materials and encased to form a heat sink product, and more particularly to a process for producing them.
There are currently many applications that require the storage of large quantities of heat for either cooling or heating an object. Typically these applications produce heat so rapidly that normal dissipation through cooling fins, natural convection, or radiation cannot dissipate the heat quickly enough and, thus, the object over heats. To alleviate this problem, a material with a large specific heat capacity, such as a heat sink, is placed in contact with the object as it heats. During the heating process, heat is transferred to the heat sink from the hot object, and as the heat sink's temperature rises, it “stores” the heat more rapidly than can be dissipated to the environment through convection. Unfortunately, as the temperature of the heat sink rises the heat flux from the hot object decreases, due to a smaller temperature difference between the two objects. Therefore, although this method of energy storage can absorb large quantities of heat in some applications, it is not sufficient for all applications.
Another method of absorbing heat is through a change of phase of the material, rather than a change in temperature. Typically, the phase transformation of a material absorbs two orders of magnitude greater thermal energy than the heat capacity of the material. For example, the vaporization of 1 gram of water at 100° C. absorbs 2,439 Joules of energy, whereas changing the temperature of water from 99° C. to 100° C. only absorbs 4.21 Joules of energy. In other words, raising the temperature of 579 grams of water from 99° C. to 100° C. absorbs the same amount of heat as evaporating 1 gram of water at 100° C. The same trend is found at the melting point of the material. This phenomenon has been utilized in some applications to either absorb or evolve tremendous amounts of energy in situations where heat sinks will not work.
Although a solid block of phase change material has a very large theoretical capacity to absorb heat, the process is not a rapid one because of the difficulties of heat transfer and thus it cannot be utilized in certain applications. However, the utilization of the high thermal conductivity foam will overcome the shortcomings described above. If the high conductivity foam is filled with the phase change material, the process can become very rapid. Because of the extremely high conductivity in the struts of the foam, as heat contacts the surface of the foam, it is rapidly transmitted throughout the foam to a very large surface area of the phase change material. Thus, heat is very quickly distributed throughout the phase change material, allowing it to absorb or emit thermal energy extremely quickly without changing temperature, thus keeping the driving force for heat transfer at its maximum.
Heat sinks have been utilized in the aerospace community to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A material that has a high heat of melting is encased in a graphite or metallic case, typically aluminum, and placed in contact with the object creating the heat. Since most phase change materials have a low thermal conductivity, the rate of heat transfer through the material is limited, but this is offset by the high energy absorbing capability of the phase change. As heat is transmitted through the metallic or graphite case to the phase change material, the phase change material closest to the heat source begins to melt. Since the temperature of the phase change material does not change until all the material melts, the flux from the heat source to the phase change material remains relatively constant. However, as the heat continues to melt more phase change material, more liquid is formed. Unfortunately, the liquid has a much lower thermal conductivity, thus hampering heat flow further. In fact, the overall low thermal conductivity of the solid and liquid phase change materials limits the rate of heat absorption and, thus, reduces the efficiency of the system.